THIS INVENTION relates to the treatment of water. More particularly, the invention relates to the treatment of water which contains carbonaceous solids. Still more particularly, the invention relates to a process for treating water which contains carbonaceous solids.
According to the invention, there is provided a process for treating water which contains carbonaceous solids, which process includes
subjecting water which contains carbonaceous solids, to hydrolysis in a biological hydrolysis reaction stage, in the presence of sulphate ions, thereby to produce treated water, a slurry component, and a sulphur-containing component; and
withdrawing treated water, the slurry component, and the sulphur-containing component from the reaction stage.
The process envisages that the water will normally be fed continuously into the reaction stage, and that treated water will normally be withdrawn continuously from the reaction stage, while the slurry component and the sulphur-containing component can be withdrawn either continuously or intermittently, depending on the reaction stage design and the build-up of these components n the reaction stage.
The carbonaceous solids in the water may be pre-settled or thickened, prior to subjecting the water to the biological hydrolysis reaction stage. The water may thus be in the form of a concentrate.
In the reaction stage, biological reduction of the sulphate ions takes place, so that the sulphate ions are converted to sulphides. The carbonaceous solids-containing water has the effect of adding metabolizable carbon to the reaction stage, for metabolization by the organisms involved in the biological sulphate reduction. The metabolizable carbon source may comprise an organic carbon source which exhibits a high chemical oxygen demand (xe2x80x98CODxe2x80x99). In one embodiment of the invention, the carbonaceous solids-containing water may be a waste water. It may thus be an effluent or waste product comprising organic material dissolved, suspended and/or carried in waste water, such as sewage, e.g. primary sewage sludge or secondary sewage sludge; settled sewage; settled sewage solids; tannery waste water; brewery waste water; starch manufacture waste water; winery waste water; and paper pulp waste water. In another embodiment of the invention, the carbonaceous solids-containing water may be water in which the carbonaceous solids are suspended, such as water containing fine lignocellulosic material. Such waters provide metabolizable organic carbon and the necessary organisms for biological sulphate reduction in the reaction stage.
The carbonaceous solids-containing water may naturally contain the necessary sulphate ions for the required biological sulphate reduction to take place in the reaction stage. However, it is envisaged that the water will normally be deficient in sulphate ions, and the process may thus include adding a source of sulphate ions to the carbonaceous solids-containing water ahead of the reaction stage and/or to the reaction stage itself. In principle, any convenient source of sulphate ions can be used.
Thus, in one embodiment of the invention, the source of sulphate ions may be mine effluent or industrial waste water. Such water normally also contains dissolved heavy metal cations, such as ferrous cations, together with dissolved sulphate anions. The process may then include pretreating the mine effluent or industrial waste water to remove the heavy metals therefrom, before adding it to the carbonaceous solids-containing water or to the reactor as pretreated sulphate-containing waste water. The pretreatment may comprise adding a sulphide to the mine effluent or industrial waste water, with the sulphide reacting with heavy metal(s) in the mine effluent or industrial waste water, thereby to cause precipitation of the heavy metal(s) from the mine effluent or industrial waste water as metal sulphides; and separating the precipitated metal sulphides from the waste water, to obtain the pretreated sulphate-containing waste water, which is then used in the process.
In the pretreatment of the mine effluent or industrial waste water, the sulphide that is added thereto may be in liquid or gaseous form. For example, it may be in the form of hydrogen sulphide.
The separation of the precipitated metal sulphides from the pretreated sulphate-containing waste water will thus be effected in a separation stage, which may comprise a settler.
However, in another embodiment of the invention, any other sulphide-containing waste water can be used as the source of sulphate ions. In yet another embodiment of the invention, one or more sulphate salts can instead, or additionally, be used as the source of sulphate ions.
The biological hydrolysis reaction stage may, in particular, comprise an accelerated hydrolysis reactor in which, as the carbonaceous solids-containing water flows along the reactor from one end thereof to the other, hydrolysis of the solids component thereof into non-digestible or refractory COD material, hereinafter also referred to as xe2x80x98RefCOD materialxe2x80x99; slowly biodegradable COD material, hereinafter also referred to as xe2x80x98SBCOD materialxe2x80x99, and readily biodegradable COD material, hereinafter also referred to as xe2x80x98RBCOD materialxe2x80x99, takes place. At least some of the RefCOD and SBCOD material has a larger particle size than the RBCOD material, which has a particle size which is typically of the order of about 0.1 xcexcm and smaller. The RefCOD and SBCOD materials typically have particle sizes in the range 60 to 100 microns. Thus, at least some of the RefCOD and SBCOD material thus settles to the bottom of the reactor as the water passes along the reactor, to be withdrawn as the slurry component. Typically, substantially all of the RefCOD and SBCOD material settles to the bottom of the reactor. At least some of the RBCOD material is withdrawn from the reactor as part of the treated water. The treated water, which thus contains solubilized/suspended material, may then be subjected to known dissolved/suspended solids treatment. Alternatively, solubilized organic material in the treated water can be used as a carbon source for nutrient removal or for tertiary water treatment operations such as nitrate (or N) and phosphate (or P) removal.
The accelerated hydrolysis reactor may comprise, at or in its bottom or base, a plurality of valleys in which the settled material collects.
The settled material can then be recycled to the reactor, preferably with shearing thereof, e.g. by means of a high shear pump. In this fashion, in addition to the hydrolysis, fractionation of the RefCOD and SBCOD material, into RBCOD material, occurs.
Typically, the accelerated hydrolysis reactor may comprise three of the valleys. The settled material from each of the valleys may be recycled to the inlet end of the reactor. Instead, however, the settled material of the second and third valleys can be recycled to the reactor downstream of its inlet end, e.g. to above the second and third valleys respectively.
At least some of the sulphides which form during the biological sulphate reduction may be in the form of gaseous hydrogen sulphide, which collects in a head space of the reactor. The withdrawal of the hydrogen sulphide as the, or as a portion of the, sulphide-containing component, may then include purging this head space with an inert gas, such as nitrogen, and withdrawing a combined hydrogen sulphide/inert gas stream from the reactor head space. Hydrogen sulphide can then be recovered from this gaseous stream. The recovered hydrogen sulphide can then typically be used as the sulphide required for precipitation of metals from the waste water. Instead, if desired, the gas stream can be subjected to sulphide oxidation, thereby to obtain sulphur or, alternatively, sulphate as a product. Alternatively, at least some of the sulphide may be added to the water to be treated, ie to the influent stream, to initiate the hydrolysis reaction.
Instead, or additionally, the redox potential in the head space of the reactor can be controlled so that oxidation of sulphide to elemental sulphur in a surface layer of water in the reactor takes place. This sulphur surface layer is then withdrawn as the, or as part of the, sulphur-containing component. The process may then include subjecting this component to separation to separate the sulphur from the water, with the water then typically being returned to the reactor. The sulphur may then be oxidized to sulphate, e.g. by means of biological sulphur oxidation, with the resultant sulphates being routed back to the reactor as at least part of the source of sulphate ions.